In recent years, various thin film solar cells have come into use, in addition to conventional amorphous thin film solar cells, crystalline thin film solar cells are also being developed, and moreover hybrid thin film solar cells obtained by laminating these solar cells together are also put in practical use.
Thin film solar cells in general comprise a first electrode, one or more semiconductor thin film photoelectric conversion units, and a second electrode laminated in an order on a substrate. And one photoelectric conversion unit comprises an i type layer sandwiched by a p type layer and an n type layer.
The i type layer is substantially an intrinsic semiconductor layer, occupies a large percentage of a thickness of the photoelectric conversion unit, and then photoelectric conversion effect is generated mainly within this i type layer. For this reason, this i type layer is usually referred to as an i type photoelectric conversion layer, or simply as a photoelectric conversion layer. The photoelectric conversion layer is not limited to an intrinsic semiconductor layer, but may be a layer obtained by being doped, within a range in which loss of light absorbed with doped impurity does not cause problems, into a p type or an n type in a very small quantity range. Although a thicker photoelectric conversion layer is more preferable for light absorption, a layer thicker than necessary may cause results of increasing cost for film-forming and time for production.
On the other hand, conductivity type layers of a p type or an n type exhibit function to generate a diffusion potential in a photoelectric conversion unit, a magnitude of this diffusion potential influences a value of an open circuit voltage as one of important characteristics of a thin film solar cell. However, these conductivity type layers are inert layers not directly contributing to photoelectric conversion, and thus light absorbed with impurity doped in the conductivity type layer gives loss not contributing to generation of electric power. Therefore, the conductivity type layers of the p type and the n type are preferably maintained for a smallest possible thickness within a range for generation of a sufficient diffusion potential.
Here, in the above-mentioned a pin (nip) type photoelectric conversion unit or a thin film solar cell, when a photoelectric conversion layer occupying a principal portion is amorphous, it is called an amorphous unit or an amorphous thin film solar cell, and when a photoelectric conversion layer is crystalline, it is called a crystalline unit or a crystalline thin film solar cell, regardless of whether conductivity type layers of p type and n type included therein are amorphous or crystalline.
As a method of improving conversion efficiency of a thin film solar cell, a method of laminating two or more photoelectric conversion units to obtain a tandem unit may be mentioned. In this method, a front unit comprising a photoelectric conversion layer having a larger band gap is disposed on a light incident side of a thin film solar cell, and a back unit comprising a photoelectric conversion layer having a smaller band gap is disposed in an order on a back side of the front unit, and this configuration thereby enables photoelectric conversion over a large wave range of an incident light, and realizes improvement in conversion efficiency as a whole solar cell. Among such tandem solar cells, especially a solar cell having an amorphous photoelectric conversion unit and a crystalline photoelectric conversion unit laminated together is referred to as a hybrid thin film solar cell.
For example, in a longer wavelength side, an i type amorphous silicone exhibits photoelectric conversion function in wavelength of a light up to about 800 nm, while an i type crystalline silicone can exhibit photoelectric conversion function with a light of longer wavelength of about 1100 nm. However, although a light absorption even with a sufficient thickness of about not more than 0.3 micrometers can be realized in an amorphous silicone photoelectric conversion layer having a larger light absorption coefficient, in a crystalline silicone photoelectric conversion layer having a smaller light absorption coefficient, in order to fully absorb light of a longer wavelength, the layer preferably has a thickness of about 1.5 to 3 micrometers. That is, usually a crystalline photoelectric conversion layer preferably has a thickness of about 5 to 10 times as large as a thickness of an amorphous photoelectric conversion layer.
In a monolayer amorphous silicon thin film solar cell, and also in the above-mentioned hybrid thin film solar cell, a thickness of a photoelectric conversion layer is desirably maintained as small as possible, from a viewpoint of improvement in productivity, that is, cost reduction. For this reason, generally used is a structure using what is called light trapping effect in which a disposition of a layer having a refractive index smaller than a refractive index of a photoelectric conversion layer, on a backside of the photoelectric conversion layer observed from a light incident side enables effective reflection of light of a particular wavelength. A disposition on a backside of a photoelectric conversion layer observed from a light incident side means a disposition contacting to the photoelectric conversion layer on a side of a back face, or a disposition on a side of a back face in a state of sandwiching an other layer disposed on a back face of the photoelectric conversion layer.
Japanese Patent Laid-Open No. 02-73672 official report discloses a structure of a solar cell in which a translucent first electrode, an amorphous silicon semiconductor thin film (hereinafter referred to as simply semiconductor thin film), a zinc oxide film having a thickness of less than 1200 angstroms, a non-translucent second electrode (metal electrode) are laminated in this order from a light incident side. The zinc oxide film has a function for preventing a silicide formed in an interface between the semiconductor thin film and the metal electrode increase absorption loss. Since refractive index difference exists between the zinc oxide film and the semiconductor thin film, a thickness of the zinc oxide film limited to a range of less than 1200 angstroms and preferably to a range of 300 angstroms to 900 angstroms has an effect of improving reflectance in an interface of the semiconductor thin film/the zinc oxide film. For this reason, a short-circuit current density of the solar cell and consequently a conversion efficiency improves. However, since the zinc oxide film is formed by a technique of sputtering, spraying, etc., different facilities from that for semiconductor thin film formed in general by plasma CVD methods etc. are required, leading to occurrence of problems of facility cost rise and longer production tact. Furthermore, there may occur problems that especially use of sputtering method in formation of the zinc oxide film may cause performance reduction by sputter damage to a ground semiconductor thin film. According to examples, the above-mentioned semiconductor thin film consists of a P type a-SiC:H film, a non doped a-Si:H film, and an n type a-Si:H film. In this case in order to generate sufficient diffusion potential in a non doped a-Si:H film, a thickness of an n type a-Si:H film requires 150 angstroms to 300 angstroms in general, which will not permit ignoring absorption loss of light passing through the n type a-Si:H film.
Japanese Patent Laid-Open No. 4-167473 official report discloses a structure, in a sequential order from light incident side, of a transparent electrode/one electric conductive type amorphous semiconductor layer/an intrinsic amorphous semiconductor layer/an amorphous silicon oxynitride or amorphous silicon oxide (hereinafter referred as a-SiON or a-SiO)/a metal oxide layer/a high reflective metal layer/a substrate. However, this a-SiON (a-SiO) layer is formed for prevention of increase in absorption loss by reduction of the metal oxide layer that may be obtained when forming the amorphous semiconductor layer on the metal oxide layer, and no description is disclosed that light trapping may be performed using refractive index difference between the a-SiON (a-SiO) layer and the intrinsic amorphous semiconductor layer. Specifically, in Examples, a thickness of a-SiON (a-SiO) layer set thin as 200 angstroms does not permit expectation of sufficient light trapping effect.
Japanese Patent Laid-Open No. 6-267868 official report discloses a method for forming a film of a-SiO including microcrystalline phase of silicon characterized by being based on decomposition of a source gas having not more than 0.6 of a value of CO2/(SiH4+CO2). The official report describes that this film represents a high photoconductivity not less than 10−6 S/cm, and a low absorption coefficient, and is suitable for a window layer of amorphous silicon based solar cells. However, this official report fails to describe about a refractive index of the obtained film, and fails to describe that the film can be disposed on a backside of a photoelectric conversion layer of the solar cell observed from a light incident side. The present inventors carried out investigation for application of a silicon oxide layer by a high frequency plasma CVD method for an n type layer of pin type silicon based thin film solar cell using SiH4, CO2, H2, and PH3 as reactive gas, based on teachings obtained by the documentary materials. As a result, it was found our that using a technique of disposing a silicon oxide layer on a backside of a photoelectric conversion layer, and of setting a ratio of CO2/SiH4 larger, light trapping effect was exhibited and a short-circuit current of the solar cell was increased when increasing an amount of oxygen in the layer and a difference of refractive index with the photoelectric conversion layer. However, only simple use of the silicon oxide as an n type layer increased a series resistance of the solar cell, leading to a problem of reduction of conversion efficiency. This is considered to originate in a contact resistance between silicon oxide and a metal oxide layers, such as ZnO as a part of a back electrode.
Thus, conventional technique cannot solve a problem of series resistance of solar cells that is believed to be caused by a contact resistance generated between a silicon based low-refractive index layer represented by silicon oxides, and a back electrode.